1. Field of the Invention
This invention relates to a hotmelt adhesive with a melting point of at least 40° C. which contains either a polymer having at least one functional group reactive to a compound with an acidic hydrogen atom and a functional group polymerizable by exposure to UV light or to electron beams or a polymer having at least one functional group reactive to a compound with an acidic hydrogen atom but no functional group polymerizable by exposure to UV light or electron beams and a compound with a functional group polymerizable by exposure to UV light or electron beams and a molecular weight (Mn) of less than 5,000.
2. Discussion of the Related Art
The machine production of laminates, particularly laminated films, is often carried out in practice by lamination with solvent-containing adhesives. Unfortunately, this is attended by various disadvantages.
If solvent-containing adhesives are used for lamination, considerable quantities of solvent generally have to be evaporated during lamination which involves high energy consumption. In addition, the waste air accumulating during evaporation of the solvent has to be purified at considerable expense in order to avoid the discharge of solvent vapors into the atmosphere. In addition, solvent-containing adhesives have the disadvantage that, as a rule, they only develop adequate strength after passing through a drying stage, i.e. after at least the predominant quantity of solvent present in the adhesive has been evaporated.
On the other hand, however, the processability of an adhesive is seriously affected by the absence of solvent. Adhesives suitable for the production of laminates are intended first and foremost to have a suitable application or processing viscosity, but to release only minimal quantities of readily volatile substances into the environment. In addition, adhesives of the type in question are generally expected to have sufficiently good early adhesion immediately after application to at least one of the materials to be joined after they have been fitted together so that the bonded materials are prevented from shifting relative to one another. In addition, however, a corresponding bond is also expected to be sufficiently flexible to withstand the various tensile and offset yield stresses to which the laminate—still at the processing stage—is generally exposed without damage to the adhesive bond or to the bonded material.
The early adhesion of the bonded materials has to satisfy particularly stringent requirements when not only thin films, but also materials which, although showing increased tensile strength, also have much greater flexural rigidity, for example sheet-form plastics with a thickness of more than about 100 μm or laminates which contain, for example, a paperboard layer and which, in general, are also more than 100 μm thick, are laminated to one another. With laminates such as these, the adhesive bond is exposed to particularly severe stressing because even light bending forces are transmitted virtually unweakened to the bond through the high flexural rigidity of the laminate. In general, conventional adhesives, because of their poor early adhesion, are unable to withstand the strong forces occurring at the bond without damage, even shortly after application.
Besides excellent early adhesion, various applications, particularly in the packaging of foods, make other demands on the quality of the adhesive bond. Thus, after curing, the adhesive bond is expected to show such high strength that packaged foods, for example, withstand without damage the increased stresses to which they are exposed, for example, during transportation or sale or by the user. In addition, the adhesive bonds in question are expected to show excellent heat resistance because foods are often packaged while they are warm or even hot with temperatures of up to about 100° C. If the adhesive bond of a food pack is not sufficiently heat-resistant in such cases, it can be damaged during the packaging process or during the cooling phase of the food, with the result that, for example, the food leaks from the pack. However, even minute cracks in the adhesive bond can be harmful to the food intended for sale, for example by allowing microorganisms to penetrate into the pack and to spoil the food.
In general, a fundamental disadvantage of the conventional solventless adhesives known from the prior art is that the adhesion properties of the adhesive after application are unsatisfactory on account of the low viscosity, so that the adhesive bond must not be exposed to any stresses before final curing to ensure that the laminate retains the intended shape. Such adhesives are generally unsuitable for the production of laminates with increased flexural rigidity. In addition, the adhesives in question generally require long cure times which often makes the production of laminates using such adhesives uneconomical.
One proposal for avoiding the disadvantages described above was to use an adhesive system hardening in several stages in the production of laminates. The adhesives used in this case were subjected in a first stage to a first rapid curing reaction by irradiation. The strength of the adhesive bond after this first curing reaction is said to be such that it enables the bonded articles or materials to be handled without difficulty. In a second curing stage, the adhesive then continues to harden until it has the required ultimate strength.
DE-A-29 13 676, for example, discloses a process for the production of film laminates using solventless adhesives. This document describes a solventless adhesive liquid at room temperature which consists of oligomeric and/or polymeric esters and/or ethers containing both free isocyanate groups and free (meth)acrylate groups in one molecule.
Unfortunately, this process is attended by the disadvantage that, although the strength of the bond is sufficient for bonding thin, flexible materials with minimal flexural rigidity, early adhesion is generally not sufficient for firmly bonding laminates of relatively thick, stiff materials in the early phase.
EP-B 0 564 483 describes reactive contact adhesives, processes for their production and their use. More particularly, this document describes urethane-based coating compositions polymerizable in two stages which, through the presence of UV-polymerizable acrylate groups, can be cured in a first curing stage to form a hard, but still formable or embossable material which then undergoes irreversible hardening in a following second stage. Monofunctional acrylates are added to the adhesive to lower its viscosity. The described adhesive has contact tackiness after irradiation. The bonding of wood and/or plastic parts at up to about 70° C., preferably at room temperature, is mentioned as one application of the described contact adhesive.